This invention relates to optical fiber communications networks and more particularly to optical fiber communications networks which use coherent technology.
In implementing an optical fiber loop distribution network, the star architecture and the bus architecture are the two basic network topologies that are generally considered. In an optical fiber network employing the star architecture, such as a metallic wire telephone distribution network, direct links are established between each end user and a transmitting end. Advantageously, since a direct path exists between the transmitting end and each user, each user receives messages from the transmitting end having approximately the same signal power with variations being due to differences in signal attenuation on the various fiber link lengths. In addition, on a star network, messages directed from the transmitting end to specific users can be privately transmitted, and, by incorporating switching capabilities at the transmitting end, complete interconnectivity between users can be achieved. Disadvantageously, an optical network having a star architecture is initially expensive to implement since a dedicated optical fiber must be installed between the transmitting end and each potential customer when service is instituted.
The bus architecture is similar to present metallic CATV systems in that a bus or trunk connected to the transmitting end signal source is tapped along its path in order to provide service to each customer. Unlike a network employing a star architecture, a network employing a bus architecture has a low penetration cost since additional taps can be added as the customer demand grows. The need for a fiber network that has such a low penetration cost is critical in providing services that are cost competitive with services provided over presently installed metallic coaxial CATV systems. Disadvantageously, the fiber optic bus network does not offer the same privacy protection offered in a star network since, without electronic or wavelength filtering, each user receives every downstream transmission. A more significant shortcoming of a fiber bus network is that there is a limit to the number of users that can share the transmission medium. As additional customers are tapped onto the trunk, the power received by some stations may fall below the minimum power constraint of the receiver and a receiver far downstream will be incapable of detecting the transmitted signal.
The detectability of any signal optically modulated for fiber transmission is determined in part by the modulation and detection techniques employed. Whereas direct detection technology is the most common technology currently employed in receiving devices, direct detection receivers are substantially less sensitive than receivers employing coherent detection technology. Although coherent detection technology is not yet mature, one can readily speculate that low cost, high quality coherent devices will be available in the near future. An optical fiber loop distribution network using a bus topology and employing coherent communications technology would therefore be the most desirable arrangement from the standpoint of the cost effectiveness of initial penetration and from the standpoint of receiver sensitivity to decreased power levels as the number of users connected to the network increases. Furthermore, as the network matures and the signal load on the trunk increases, coherent technology is readily adaptable to very dense wavelength division multiplexing. Thus onto a single fiber, which initially may carry signals at a single wavelength, can be "piggy-backed" a plurality of signals transmitted at a plurality of wavelengths. Disadvantageously, however, a local oscillator is required at each coherent detection receiver which is both expensive and adds complexity to the system.
An object of the present invention is to provide an optical fiber loop distribution point-to-multipoint network that has the cost efficiency of a bus network and which permits a significant number of users to share a common optical fiber facility.
An additional object of the present invention is to provide an optical fiber loop distribution point-to-multipoint network using coherent detection technology in such a way that obviates the necessity of a local oscillator at each receiver.
A feature of the optical fiber loop distribution network of the present invention is that it can readily mature into a network capable of handling additional information payloads as the demand increases.
An additional feature of the present invention is that it can be employed to develop a mature two-way communications network.